As a method for obtaining a large screen image, it is well known conventionally that an optical image is formed on a reflection type light valve in accordance with a video signal, and the optical image is irradiated with light to be projected on a screen in an enlarged state by a projection lens system. If a reflection type light distribution correction element (reflection type light valve) for forming an image by controlling the traveling direction of light in accordance with a video signal is used, a projected image with a more efficient light use and a higher brightness can be displayed.
FIG. 6 shows an example of an optical system of a conventional projection type display apparatus using a reflection type light valve. As shown in FIG. 6, a light source 1 is composed of a lamp 1a and a concave mirror 1b. The concave mirror 1b is a parabolic mirror, which is formed by vapor-depositing an optical multilayer film that transmits infrared light and reflects visible light on an inner surface of a glass substrate. The lamp 1a is disposed so that the center of its illuminator is positioned at a focal point f1 of the concave mirror 1b. The light radiated from the lamp 1a is reflected from the concave mirror 1b and travels to an illumination optical system 2.
The illumination optical system 2 is composed of a first lens array 3 and a second lens array 4. The first lens array 3 and the second lens array 4 respectively are composed of a plurality of lens elements. The first lens array 3 and the second lens array 4 are placed so that each lens element constituting the first lens array 3 forms an illuminator image on each corresponding lens element of the second lens array 4 with light incident upon the illumination optical system 2. The light incident upon the illumination optical system 2 is split into a plurality of luminous fluxes by the first lens array 3 and the second lens array 4. Thereafter, the luminous fluxes are overlapped with each other by a field lens 5 to be incident upon a polarization beam splitter 6.
The light incident upon the polarization beam splitter 6 has only a light beam in a particular polarization direction reflected, and this is incident perpendicularly upon a display region of a reflection type light valve 7. The reflection type light valve 7 is a reflection type liquid crystal panel, and changes a reflectance of polarized light in accordance with a video signal to form an optical image. Output light 9 from the reflection type light valve 7 has a polarization direction rotated by 90° with respect to the incident light 8 to be output. The output light 9 is incident upon a projection lens system 10 after passing through the polarization beam splitter 6, and an optical image on the reflection type light valve 7 is projected onto a screen (not shown) by the projection lens system 10.
FIG. 7 shows another example of an optical system of a conventional projection type display apparatus using a reflection type light valve. The light source 1, the illumination optical system 2, and the field lens 5 shown in FIG. 7 are the same as those shown in FIG. 6. The light output from the field lens 5 passes through a relay lens system 21, is reflected from a mirror 22, and passes through a projection optical system back group 23 to illuminate the reflection type light valve 24.
The reflection type light valve 24 is a reflection type liquid crystal panel similar to that shown in FIG. 6, which changes a reflectance of polarized light in accordance with a video signal to form an optical image. A projection lens system 25 is composed of a projection optical system front group 26 and the projection optical system back group 23. The optical image on the reflection type light valve 24 is projected on a screen (not shown) in an enlarged state by the projection lens 25. In the projection type display apparatus, illumination light illuminates the reflection type light valve 24 in a direction tilted from the normal direction to the display region of the reflection type light valve 24. Therefore, the illumination light 27 and projected light 28 do not share an optical path.
As the reflection type light valve, a DMD (Digital Micro Mirror Device) is being given attention. The DMD has a configuration in which a plurality of minute reflection mirrors (hereinafter, referred to as “micro mirrors”) are disposed two-dimensionally on a silicon substrate, and each micro mirror constitutes a pixel. Each micro mirror is configured so as to move like a seesaw in a range of ±10° by two rotation spindles provided in a diagonal direction at a diagonal position of a pixel. For example, it is assumed that the state where a micro mirror is tilted at +10° is ON, and the state where a micro mirror is tilted at −10° is OFF. The DMD tilts each micro mirror at +10° or −10° in accordance with a video signal, thereby controlling the output direction of a light beam to form an optical image.
FIG. 8 shows an operation state of micro mirrors constituting the respective pixels of a conventional DMD. FIG. 8 shows a cross-section taken along a surface perpendicular to a rotation spindle of each micro mirror of the DMD. The counterclockwise direction corresponds to a rotation positive direction of a micro mirror. In FIG. 8, reference numerals 31 to 36 denote micro mirrors that constitute the respective pixels. Reference numeral 37 denotes a part of a projection lens system.
In the example shown in FIG. 8, the micro mirrors 31, 33, and 36 are tilted at +10° (n a counterclockwise direction) with respect to a reference surface 38 of the reflection type light valve (DMD), whereby they are in an ON state. Therefore, incident light 39 reflected from the micro mirrors 31, 33, and 36 is incident upon the projection lens system 37. On the other hand, the micro mirrors 32, 34, and 35 are tilted at −10° (in a clockwise direction) with respect to the reference surface 38 of the reflection type light valve, whereby they are in an OFF state. Therefore, the incident light 39 reflected from the micro mirrors 32, 34, and 35 is not incident upon the projection lens system 37. Such a DMD has useful characteristics, i.e., it can use natural light, and has a high light use efficiency and a high response speed, compared with a liquid crystal panel using polarized light.
WO 98-29773 shows a configuration example of an optical system of a projection type display apparatus using a DMD as a reflection type light valve. FIG. 9A shows a schematic configuration of a projection type display apparatus using a conventional DMD. FIG. 9B shows a portion in the vicinity of the reflection type light valve in an enlarged state. FIG. 9A shows a cross-section taken along a surface perpendicular to a rotation spindle of each micro mirror of the reflection type light valve 46.
First, a description will be made with reference to FIG. 9A. A light source 11 is composed of a concave mirror 11b and a lamp 11a in the same way as the light source 1 shown in FIG. 6. The concave mirror 11b is the same as the concave mirror 1b shown in FIG. 6 except that the concave mirror 11b is an elliptical mirror. The concave mirror 11b is formed by vapor-depositing an optical multilayer film that transmits infrared light and reflects visible light on an inner surface of a glass substrate. The lamp 11a is disposed so that the center of its illuminator is positioned at a first focal point (not shown) of the concave mirror 11b. 
Light radiated from the lamp 11a is reflected from the concave mirror 11b, and travels to a second focal point (not shown) of the concave mirror 11b. The light output from the lamp 11a forms an illuminator image at the second focal point of the concave mirror 11b. The light passing through the second focal point is incident upon an illumination optical system 12 and is split into a plurality of luminous fluxes. Thereafter, the luminous fluxes are incident upon a relay lens 15 to be overlapped with each other. The illumination optical system 12 is configured in the same way as the illumination optical system 2 shown in FIG. 6.
The light output from the relay lens 15 is reflected from a total reflection mirror 41 to be incident upon a total reflection prism 43 via a field lens 42. The total reflection prism 43 is composed of two single prisms 43a and 43b separated by an air layer 44. Reference numeral 48 denotes a projection lens system.
Next, a description will be made with reference to FIG. 9B. Incident light 45 that is incident upon the total reflection prism 43 is totally reflected from an interface between the single prism 43b and the air layer 44 to travel to the reflection type light valve 46. The reflection type light valve 46 controls the traveling direction of light in accordance with a video signal to form an optical image. Reflected light 47 from the reflection type light valve 46 is output as luminous fluxes having a principal ray perpendicular to a display region of the reflection type light valve 46, passes through the total reflection prism 43 without being reflected from the interface between the single prism 43b or 43a and the air layer 44, and is incident upon the projection lens system 48 (see FIG. 9A). As a result, the optical image on the reflection type light valve 46 is projected on a screen in an enlarged state by the projection lens system 48.
However, in the projection type display apparatus shown in FIG. 6, the reflection type liquid crystal panel used as the reflection type light valve 7 has characteristics of reflecting illumination light, which is incident upon a substrate surface perpendicularly, in a direction perpendicular to the substrate surface. Therefore, the illumination light (incident light 8) and the projected light (output light 9) pass through substantially the same optical path in opposite directions. This makes it necessary to provide means for separating the incident light 8 from the output light 9, such as the polarization beam splitter 6 as shown in FIG. 6. The polarization beam splitter 6 is composed of a large glass block and a multilayer film. Therefore, in the projection type display apparatus shown in FIG. 6, there is a problem of high cost.
Furthermore, the polarization beam splitter 6 separates the output light 9 from the reflection type light valve 7, from the incident light 8 based on the direction of a polarization plane. If there is nonuniformity in a medium constituting a prism, unnecessary light components are generated to decrease a contrast. Furthermore, the polarization beam splitter 6 separates only components having different polarization directions. Therefore, it is necessary to previously align the polarization plane of the incident light 8. Therefore, it is necessary to introduce means for aligning a polarization plane of the incident light 8, which increases cost. On the other hand, in the case where means for aligning a polarization plane is not introduced, a light use efficiency is decreased remarkably to ½ or less.
In the projection type display apparatus shown in FIG. 7, the illumination light 27 incident upon the reflection type light valve 24 and the projected light 28 output therefrom do not share an optical path. Therefore, there is an advantage that it is not necessary to provide a polarization beam splitter. However, in the projection type display apparatus shown in FIG. 7, two projection optical systems for illumination light and projected light are required, so that an F-number required in a projection optical system becomes twice that of the example in FIG. 6. This makes it necessary to enlarge a lens constituting the projection optical system, and to increase the number of lenses so as to ensure performance, leading to enlargement of an optical system and an increase in cost.
On the other hand, if the projection type display apparatus shown in FIG. 9 is used, a problem involved in optical paths of illumination light and projected light and a problem of enlargement of the projection optical system can be solved. However, the projection type display apparatus shown in FIG. 9 requires the total reflection prism 43 for separating illumination light from projected light, resulting in an increase in cost. Furthermore, the total reflection prism 43 includes a minute air layer, so that the resolution characteristics of the projection lens 48 are influenced greatly by the tolerance of the air layer.
In order to solve the above-mentioned problem, JP 2000-98272 A discloses a configuration in which a projection optical system is designed as a non-telecentric type, and illumination is generated in accordance therewith. FIG. 10A shows a schematic configuration of a conventional projection type display apparatus in which a projection optical system is designed as a non-telecentric type. FIG. 10B shows a portion in the vicinity of a reflection type light valve in an enlarged state. In the projection type display apparatus shown in FIG. 10B, a DMD is used as a reflection type light valve 63. FIG. 10B shows a cross-section taken along a surface perpendicular to a rotation spindle of each micro mirror of the reflection type light valve 63.
As shown in FIG. 10A, a light source 21 is composed of a lamp 21a and a concave mirror 21b in the same way as the light source shown in FIG. 6. The concave mirror 21b is an elliptical mirror, which is the same as the concave mirror 11b shown in FIG. 9. The lamp 21 is disposed so that the center of its illuminator is positioned at a first focal point f1 of the concave mirror 21b. In the same way as the example shown in FIG. 6, light radiated from the lamp 21 is reflected from the concave mirror 21b to form an illuminator image at a second focal point f2 of the concave mirror 21b. The light passing through the second focal point f2 is incident upon a rod lens 61 to be made uniform. The illumination light that has been made uniform by the rod lens 61 passes through a relay lens 62.
As shown in FIG. 10B, the illumination light passing through the relay lens 62 passes through an output plane 67 of an illumination optical system to be incident upon the reflection type light valve 63 at a predetermined incident angle. The reflection type light valve 63 controls the traveling direction of light in accordance with a video signal to form an optical image. The incident light 64a to 64c incident on the reflection type light valve 63 is reflected respectively at predetermined angles, and reflected light (output light) 65 is projected on a screen by a projection lens system 68 in an enlarged state. Reference numeral 66 denotes a pupil of the projection optical system.
Thus, the projection type display apparatus shown in FIG. 10 uses a projection optical system of a non-telecentric type and does not require a total reflection prism. Therefore, it is considered that the cost can be decreased more than that of the projection type display apparatus shown in FIG. 9.
However, in the configuration of the reflection type display apparatus shown in FIG. 10, the normal directions of the reflection surfaces of micro mirrors become constant over the display region of the reflection type light valve 63. Therefore, when the optical axis of the reflection type light valve 63 is substantially matched with that of the projection optical system, the optical paths of the incident light and output light are overlapped with each other. Because of this, it is physically difficult to obtain a satisfactory image with uniform illumination, and the optical axis of the projection optical system needs to be offset from that of the reflection type light valve 63 so as to separate the incident light 64 from the output light 65. Thus, the projection optical system projects an image in an axially shifted manner, which requires enlargement of an effective display region. Consequently, the optical system is enlarged, leading to an increase in cost. Furthermore, there is a problem that front projection cannot be performed.
Furthermore, JP 11(1999)-249069 A discloses a projection type display apparatus in which immediately before a display region of a reflection type light valve, a condenser lens having incidence-reflection characteristics varied depending upon the display region is placed with its optical axis deflected from that of the projection optical system. In this projection type display apparatus, a double F-number is not required in the projection optical system, and optical paths of incident light and output light are not overlapped with each other.
However, in the projection type display apparatus, the condenser lens constituting a part of the projection optical system needs to be deflected. Therefore, when it is attempted to obtain satisfactory resolution characteristics over the projected image region, a complicated lens configuration is required. Furthermore, in order to obtain a satisfactory resolution, the reflection type light valve is tilted at 2° to 8° with respect to the optical axis of the projection optical system. However, according to the “Shineproof Theorem”, it is considered that a projected image of the reflection type light valve also is tilted with respect to the optical axis of the projection optical system. Therefore, in the case where the display region of the reflection type light valve is in a rectangular shape, the projected image on a surface perpendicular to the optical axis has a trapezoidal shape; accordingly, it may be difficult to obtain a satisfactory display image.
The object of the present invention is to solve the above-mentioned problem and provide a small projection type display apparatus including a reflection type light valve, in which an optical path of incident light and that of output light can be prevented from being overlapped with each other in the reflection type light valve, and a projected image of high quality can be obtained.